Sound waves may be viewed as being generally mechanical from the standpoint that they consist of the vibration of molecules about their equilibrium positions, and they are accordingly best transmitted through solid media. Sound waves with frequencies above the upper limit audible to the human ear (about 18,000 Hz) lie in the ultrasonic range. There are two main classes of ultrasound presently in clinical use: (1) High frequency (5-7 MHZ),low-power ultrasound, which is employed extensively in diagnostic ultrasonography; and (2) Low-frequency (20 to 45 kHz), high power ultrasound which has recently been put to therapeutic use.
It has been known for some time that the application of acoustic energy or force for a stream of flowing liquid, such as blood, will have an effect upon the behavior of gas bubbles entrained therein. An article entitled “Acoustic Effects on Gas Bubbles In the Flows of Viscous Fluids and Whole Blood” appeared in the Journal of Acoustical Society of America, 53, 5, 1327-1335, I. C. Maceto and Wen-Jeo Yang (1973), which discussed the use of acoustic or ultrasonic waves to trap small bubbles against the wall of the tube in which the liquid stream is flowing, using liquids that resemble whole blood in their rheological property; it was shown that the bubbles could be deflected and trapped against the sidewall of the tube in which flow is occurring. In 1992, Schwarz, Karl Q. et al., published an article entitled “The Acoustic Filter: An Ultrasonic Blood Filter for the Heart-Lung Machine”, in the Journal of Thoracic and Cardiovascular Surgery, 104, 6, 1647-1653 (December 1992). This article indicated that microbubbles in a chamber can be pushed to the opposite end of the chamber where they can be accumulated and eventually carried through a waste port, as a result of which it would be feasible to use acoustic radiation force to filter small gas bubbles from blood, while cautioning that such ultrasonic energy might cause implosion of gas bubbles that could potentially result in blood trauma, e.g. hemolysis, and thus should possibly be avoided for such reason.
U.S. Pat. No. 5,022,899, entitled Sonic Debubbler for Liquids, discloses devices that employ an ultrasonic transducer to produce low power anisotropic sound waves at about the resonant frequencies of bubbles to drive the bubbles in a specific direction where they would be rejected by being drawn out through a fluid outlet port or trapped in a disposable open cell bubble trap. Power levels are regulated so as to remain below a level which would cause hemolysis from cavitation. U.S. Pat. No. 5,334,136 shows a system for reducing post-cardiopulmonary bypass encephalopathy due to microembolization of the brain of a patient as a result of microbubbles that may arise during open-heart surgery when a cardiopulmonary bypass machine is employed. The patient's bloodstream is subjected to an ultrasonic traveling wave which is directed across the stream of blood without reflection so as to sweep the blood clean of microbubbles without inducing blood cell trauma. The microbubbles are carried by the traveling wave to a waste exit port.
Although such early devices as those in the above-identified U.S. patents showed the principle to be sound, devices for more efficient operation have continued to be sought as well as devices that could be associated directly with the human body itself so as to have an effect upon the internal bloodstream in a patient who is undergoing treatment.